A wallet is a handy place to keep all kinds of the plastic cards, such as credit card, charge card, bank card, ID card, etc. However, these cards are often lost or stolen, thereby resulting in a great deal of inconvenience and monetary loss to the card holder. In order to prevent the loss of the cards kept in a wallet, the wallet is provided with an electrical device capable of buzzing at the time when a card is removed from the card slot of the wallet. The card slot is provided with two metal sheets which are connected with a battery and a buzzer. When the card is removed from the space between the two metal sheets, the circuit is connected with the battery to result in the buzzing of the buzzer. As the card is put back into the space between the two metal sheets, the circuit is disconnected with the battery, thereby resulting in the inactivation of the buzzer. Such a conventional wallet as described above is relatively heavy in light of the metal sheets which are contained in the card slots of the wallet. An improved wallet is provided with a printed circuit in place of the metal sheets, as shown in FIG. 1. The printed circuit comprises a plastic sheet 1 on which a sensing circuit is printed. The sensing circuit is composed of two parallel conductive circuits 2 and 3, which are formed of the electrically conductive ink and are connected with a circuit board 4 which is in turn connected with a power source as well as a buzzer 5. As shown in FIG. 2, the plastic sheet 1 is folded and then inserted into the card slots of the wallet such that the conductive circuits 2 and 3 are opposite to each other. After the prolonged use of the wallet, the printed circuit becomes unreliable in view of the fact that the plastic sheet 1 lacks elasticity, thereby resulting in the poor contact between the two conductive circuits 2 and 3. The poor contact can be also caused by the loosening of the card slots. The two prior art wallets described above share a common drawback in such a manner that the buzzer keeps making a buzzing sound as long as the card or cards are removed from the card slots, thereby causing a great deal of annoyance not only to the wallet owner but also the people around the wallet owner.